Science is commonly thought to have undercut belief in God. As Nobel Prize winning physicist Steven Weinberg famously remarked, “the more we find out about the universe, the more meaningless it all seems.” Yet, the discoveries of modern physics and cosmology in the last 50 years have shown that the structure of the universe is set in an extraordinarily precise way for the existence of life; if its structure were slightly different, even by an extraordinarily small degree, life would not be possible. In many people’s minds, the most straightforward explanation of this remarkable fine-tuning is some sort of divine purpose behind our universe.

This fine-tuning falls into three categories: the fine-tuning of the laws of nature, the fine-tuning of the constants of physics, and the fine-tuning of the initial conditions of the universe. “Fine-tuning of the laws of nature” refers to the fact that if the universe did not have precisely the right combination of laws, complex intelligent life would be impossible. If there were no universal attractive force (law of gravity), for example, matter would be dispersed throughout the universe and the energy sources (such as stars) needed for life would not exist. Without the strong nuclear force that binds protons and neutrons together in the nucleus, there would not be any atoms with an atomic number greater than hydrogen, and hence no complex molecules needed for life. And without the Pauli-exclusion principle, all electrons would fall to the lowest orbital of an atom, undercutting the kind of complex chemistry that life requires.

Some fundamental physical numbers governing the structure of the universe—called the constants of physics—also must fall into an exceedingly narrow range for life to exist. For example, many have estimated that the cosmological constant—a fundamental number that governs the expansion rate of empty space—must be precisely set to one part in 10120 in order for life to occur; if it were too large, the universe would have expanded too rapidly for galaxies and stars to form, and if it were too small, the universe would have collapsed back on itself. As Stephen Hawking wrote in his book A Brief History of Time, “The remarkable fact is that the values of these numbers [i.e. the constants of physics] seem to have been very finely adjusted to make possible the development of life.” Finally, the initial distribution of mass energy at the time of the big bang must have an enormously special configuration for life to occur, which Cambridge University mathematical physicist Roger Penrose has calculated to be on the order of one part in 1010123. This is an unimaginably small number.